Advisory Committee for Pharmaceutical Science

 

 

 

Briefing Document

 

 

 

Levothyroxine Bioequivalence

 

 

Advisory Committee Meeting – 12-13 March 2003

 

 

 

 

THIS DOCUMENT CONTAINS FULLY RELEASABLE INFORMATION

 

*  Abbott Laboratories


Table of Contents

Page

1.0       Executive Summary. 1

2.0       Thyroid Biology. 2

3.0       Assessment of the Current Guidance for LT4 Bioequivalence. 4

3.1      Background and rationale for M02-417 study. 4

3.2      Results of M02-417 study. 5

3.2.1       T4 without correcting for endogenous T4 baseline concentrations. 5

3.2.2       T4 after correction for endogenous T4 baseline concentrations. 6

3.2.3       Other observations. 6

3.3      Conclusions from M02-417 study. 7

4.0       LT4 Therapy and the Clinical Consequences of Under- or  Over-Treatment 8

4.1      TSH is the measurement of adequacy of treatment 8

4.2      LT4 therapy is individualized and carefully titrated. 9

4.3      Fine-dosing increments – importance and medical use. 10

4.4      Clinical consequences of hypothyroidism and hyperthyroidism.. 12

4.4.1       Patient populations treated with LT4 products. 12

4.4.2       Consequences of hypothyroidism and hyperthyroidism.. 12

4.5      Summary of LT4 therapy and clinical consequences. 14

5.0       Conclusions. 14

6.0       References. 16

 

 

 

 

 

 

List of Figures

Figure 1.         Basic Schematic of the Thyroid Hormone System.. 3

Figure 2.         The Relationship Between Serum TSH And Free T4 Concentrations in Individuals With Stable Thyroid Status and Normal Hypothalamic-Pituitary Function. 4

Figure 3.         Resultant TSH levels With Incremental 25 µg Changes in LT4 Dosage. 11

 

                                                                                                                                               List of In-Text Tables 

Table 1.           Bioequivalence and Relative Bioavailability–Uncorrected Levothyroxine (T4)     5

Table 2.           Bioequivalence and Relative Bioavailability for T4 (Correction Method 3)           6

 

 

Appendix A     Abbott Laboratories. Study M02-417 Synopsis and Discussion............ 19

 


1.0                      Executive Summary

The purpose of this document, and Abbott's participation in the March 13, 2003 Pharmaceutical Science Advisory Committee meeting, is to identify and discuss scientific issues related to the bioequivalence assessment criteria of levothyroxine sodium (LT4) products.

Levothyroxine or thyroxine (T4) is an endogenous hormone secreted from the thyroid gland and is subject to complex biologic regulation.  As such, it has characteristics different from drugs for which there are no endogenous levels.  Exogenously administered LT4 hormone is indistinguishable from endogenously secreted T4, both in its physiologic effects and its quantification as measured in blood.  The current FDA guidance for assessment of bioequivalence of administered LT4 products does not take into account the contribution of endogenous T4.  The presence of endogenous T4 and its dynamic regulation confound the assessment of bioequivalence of LT4 products in healthy normal subjects, and consequently, preclude any conclusions about their therapeutic substitution in patients. 

We describe a recent study conducted by Abbott Laboratories that highlights these issues with the current FDA Guidance for assessing bioequivalence of LT4 products.  The study demonstrates that, following the current FDA criterion for levothyroxine sodium products, the use of T4 pharmacokinetic parameters uncorrected for endogenous T4 would result in declaring two products bioequivalent when they actually differ in drug content by as much as 33%.  Considering the margin by which the conditions for declaring bioequivalence were passed in this study, products that differ by even more than 33% would also have a high likelihood of being declared bioequivalent.  Three methods of correction for endogenous T4 levels were evaluated, but none of the methods could discern products that differ by 12.5%; dosage changes of such magnitude are clinically important.

The clinical relevance of a 12.5% difference in dose is substantiated by product labeling, standard medical management of thyroid patients, and data from clinical studies.  In class labeling for all LT4 products, it is recommended that titration be done in 12.5 to 25 µg increments for elderly patients with cardiac disease; who represent a significant number of the 13 million LT4-treated patients in the U.S.  In fact, the FDA has explicitly recognized the clinical relevance of these dosage increments particularly with respect to patient safety.  In addition, physicians employ a large number of dosage strengths to effectively titrate patients to normal thyroid status, which generally requires a dose between 100 and 150 µg.  In this dose titration process, 12-13 µg is the second most commonly used dose increment or decrement.  Finally, mildly abnormal thyroid function, which may result from slight under- or over-dosing, has been demonstrated in clinical studies to have adverse effects on fetal development, lipids, and cardiovascular disease.  For patients with thyroid cancer there is an extra concern, in that slight under-treatment increases the risk of cancer recurrence and metastatic disease.

Careful consideration should be given to developing a specific guidance for the assessment of bioequivalence of levothyroxine sodium products.  This guidance must adequately consider the unique nature of the thyroid hormone system and the demonstrated limitations of the current guidance not adequately remedied by simple methods for baseline correction. 

 

2.0                      Thyroid Biology

Thyroxine (T4) is an endogenous molecule that is synthesized and released from the thyroid gland in response to thyrotropin or thyroid-stimulating hormone (TSH) (Figure 1).  T4 is a “pro-hormone” that is converted via deiodination in tissues to triiodothyronine (T3), the most biologically potent form of thyroid hormone.  Thyroid hormones (T4 and T3) affect protein, lipid, and carbohydrate metabolism, growth, and development.  They stimulate the oxygen consumption of most cells of the body, resulting in increased energy expenditure and heat production, and possess a cardiac stimulatory effect that may be the result of a direct action on the heart.  Thyroid hormones, T4 and T3, are specifically bound by three different plasma transport proteins, each with its specific affinity and capacity for T4 and T3.  T3 controls the transcription of numerous genes that are vital to growth and development.  With thyroid hormone receptors in virtually every tissue in the body, thyroid hormone affects, via control of specific genes, proper brain development (myelin basic protein gene) and growth (growth hormone gene), and muscle function (myosin heavy chain gene, sarcoplasmic reticulum ATPase gene) and cholesterol levels (LDL-receptor gene). 1, 2 


The thyroid hormone system is under the tight feedback regulation by the hypothalamic-pituitary axis, which senses the levels of T3 and T4, and modulates the release of hypothalamic thyrotropin-releasing hormone (TRH) and pituitary TSH (Figure 1). 

Figure 1.     Basic Schematic of the Thyroid Hormone System

The pituitary is the key “biosensor” in the feedback loop, with the magnitude of TSH release controlled primarily by blood levels of T4 and T3 and some “fine-tuning”contributed by the TRH level.  Figure 2 demonstrates the inverse relationship between TSH and free T4 levels, as observed in over 500 ambulatory subjects. 3   

Linear regression analysis of the data points demonstrates that, for every 2-fold change in free T4, the TSH level will change 100-fold (refer to triangle on the graph).  Thus, TSH is considered to be the most sensitive measure of thyroid function, and is used clinically for the diagnosis and monitoring of thyroid patients.  In fact, the diagnosis of hypo- or hyperthyroidism rests on the finding of an abnormal TSH, which is more sensitive than an abnormal T4.  Furthermore, titration of LT4 dosage is performed by monitoring TSH levels, and a euthyroid state is considered achieved when TSH levels move to within the normal range (see Section 4.1).


Figure 2.     The Relationship Between Serum TSH And Free T4 Concentrations in Individuals With Stable Thyroid Status and Normal Hypothalamic-Pituitary Function.

In summary, thyroid hormones are produced by the thyroid gland and regulated by a complex control system such that, in healthy subjects with normal thyroid function (euthyroid), T4 and T3 are tightly controlled within narrow ranges. 

 

3.0                      Assessment of the Current Guidance for LT4 Bioequivalence

3.1            Background and rationale for M02-417 study

Evaluation of the pharmacokinetic curves generated for the NDA filings of LT4 products led Abbott to question the sensitivity of bioavailability studies done in healthy volunteers with no adjustment made for the endogenous baseline concentrations of T4.  We hypothesized that given the magnitude of the endogenous T4 measured at baseline, LT4 products with large differences in bioavailability could be declared bioequivalent if this method were used.  The current FDA bioequivalence methodology is to evaluate pharmacokinetic (PK) parameters using healthy volunteers, comparing 600 µg of the test compound to 600 µg of the reference compound in a crossover study, without correction for endogenous T4 baseline level. 4  Abbott conducted a “bioequivalence” study in healthy volunteers using known dosages of a single formulation of LT4 (Synthroid®) to test the sensitivity of the current FDA Guidance.  We evaluated if the current methodology was able to differentiate two known lower dosages (400 and 450 µg) from the reference dose of 600 µg.  We went on to evaluate the impact of various methods of correcting for endogenous T4 baseline on the bioequivalence assessment in this study.

3.2            Results of M02-417 study

Results for bioequivalence assessment are presented below for 400 µg versus 600 µg, 450 µg versus 600µg, and 450 µg versus 400 µg, using PK parameters uncorrected for baseline T4 levels and corrected for baseline T4 levels are listed below. 

3.2.1              T4 without correcting for endogenous T4 baseline concentrations

The relative bioavailabilities for the 450 µg and 400 µg doses as compared to the reference dose of 600 µg, using PK parameters (Cmax and AUC48) of T4 without correction of the baseline are listed in Table 1.  In addition, the relative bioavailability of 450 µg compared to the 400 µg is listed.

Table 1.     Bioequivalence and Relative Bioavailability–Uncorrected Levothyroxine (T4)

Regimens

 

 

 

Relative Bioavailability

Test vs.

Pharmacokinetic

Central Value*

Point

90% Confidence

Reference

Parameter

Test

Reference

Estimate+

Interval

450 µg vs.600 µg

Cmax

13.0

14.0

0.928

0.890 – 0.968

 

AUC48

481.7

504.8

0.954

0.927 – 0.982

400 µg vs. 600 µg

Cmax

12.9

14.0

0.921

0.883 – 0.960

 

AUC48

469.6

504.8

0.930

0.904 – 0.958

450 µg vs. 400 µg

Cmax

13.0

12.9

1.007

0.967 – 1.050

 

AUC48

481.7

469.6

1.026

0.997 – 1.055

*    Antilogarithm of the least squares means for logarithms.

+   Antilogarithm of the difference (test minus reference) of the least squares means for logarithms.

Bioequivalence is concluded for each of the comparator pairs (450 µg versus 600 µg; 400 µg versus 600 µg and 450 µg versus 400 µg) because the 90% confidence intervals from the analyses of the natural logarithms of Cmax and AUC48 are within the 0.80 to 1.25 range.

3.2.2              T4 after correction for endogenous T4 baseline concentrations

Three methods of correction were evaluated.  These three methods are defined in Appendix A, Criteria for Evaluation.  The relative bioavailabilities for the 450 µg and 400 µg doses as compared to the reference dose of 600 µg, using PK parameters (Cmax and AUC48) of T4 with correction of the baseline (Correction Method 3) are listed in Table 2.  The relative bioavailability of 450 µg compared to the 400 µg is also listed.

Results using Correction Method 3 are listed here because the point estimates for relative bioavailability as defined by AUC48 were generally further from unity than were the point estimates for that parameter using Corrections Method 1 and 2.  The results using the other correction methods are listed in the expanded summary of the M02-417 study (see Appendix A for details).  The determination for bioequivalence did not differ, no matter which correction method was used.

Table 2.     Bioequivalence and Relative Bioavailability for T4 (Correction Method 3)

Regimens

 

 

 

Relative Bioavailability

Test vs.

Pharmacokinetic

Central Value*

Point

90% Confidence

Reference

Parameter

Test

Reference

Estimate+

Interval

450 µg vs.600 µg

Cmax

5.7

6.9

0.820

0.757 – 0.888

 

AUC48

125.1

172.9

0.723

0.672 – 0.779

400 µg vs. 600 µg

Cmax

5.3

6.9

0.775

0.715 – 0.839

 

AUC48

115.4

172.9

0.667

0.620 – 0.718

450 µg vs. 400 µg

Cmax

5.7

5.3

1.058

0.979 – 1.145

 

AUC48

125.1

115.4

1.084

1.008 – 1.165

*    Antilogarithm of the least squares means for logarithms.

+   Antilogarithm of the difference (test minus reference) of the least squares means for logarithms.

These analyses indicate that the use of baseline corrected T4 pharmacokinetic parameters allow 400 and 450 µg to be differentiated from 600 µg.  However, analyses using these simple methods of correction, each method limited by its inherent assumptions, failed to distinguish 450 µg from 400 µg.

3.2.3              Other observations

Analysis of the T4 concentration data obtained during the 24 hours prior to the administration of the PK dose for each period confirmed that T4 has a diurnal cycle.  Likewise, the serum concentrations of TSH showed a clear diurnal variation for Study  Day –1 of each period.  Administration of all three doses had homeostatic effects, but did not completely suppress the serum TSH concentration during the 24 hours following the PK dose.  Analyses of the AUC24 for Study Day –1 revealed that the regimens (dose levels) had statistically significant different carryover effects from one period to the next (first-order carryover) and from Period 1 to Period 3 (second-order carryover).

3.3            Conclusions from M02-417 study

First, the results indicate that the use of baseline uncorrected T4 pharmacokinetic parameters would result in declaring two products bioequivalent when they actually differ by as much as 25% to 33% (450 µg and 400 µg versus 600 µg).  Considering the margin by which the conditions for declaring bioequivalence were passed in this study, products that differ by even more than 33% would also have a high likelihood of being declared bioequivalent products, stemming from the significant and complex contribution of endogenous T4. 

Second, the results from this study indicate that the use of baseline corrected T4 pharmacokinetic parameters would reduce the likelihood that two products would be declared bioequivalent when they actually differ by 25% to 33%.  However, analyses using three simple methods of correction, each method limited by its inherent assumptions, failed to distinguish 450 µg from 400 µg.  This is a 12.5% difference which, when applied to the range of doses typically used in clinical practice, is a clinically significant difference, as reflected in product labeling, clinical usage, and data from clinical studies. 

Furthermore, it is apparent that simple methods of correction for endogenous T4 concentrations in healthy volunteers are inadequate since these concentrations not only fluctuate on a diurnal cycle but may also be differentially affected by products with different rates and extents of absorption.  Additionally, there is evidence of significant carryover from one dosing period to subsequent periods even with washout periods up to 53 days. 

This study illustrates important flaws in the design and analysis of single-dose crossover studies in healthy volunteers to assess bioequivalence of LT4 products, stemming from the significant and complex homeostatic mechanisms associated with administration of supraphysiologic doses of LT4.  We now know that better characterization of and correction for endogenous T4 is required to provide proper interpretation of results in healthy volunteer studies.  Alternative approaches to account for endogenous T4 need to be identified and investigated.  A change to the current FDA criterion beyond adding a simple correction for baseline T4 is necessary. 

4.0                             LT4 Therapy and the Clinical Consequences of Under- or  Over-Treatment

Levothyroxine sodium is the treatment of choice as replacement or supplemental hormone therapy, or to suppress pituitary TSH in the treatment of thyroid carcinomas and nodules.  According to the Dosage and Administration section in the product labels for all levothyroxine sodium products, 5-7 “The goal of replacement therapy is to achieve and maintain a clinical and biochemical euthyroid state.  The goal of suppressive therapy is to inhibit growth and/or function of abnormal thyroid tissues.”  The fundamental guiding principle of therapy is the maintenance of TSH in the desired range by individual titration of LT4 dose.

4.1            TSH is the measurement of adequacy of treatment

Professional societies and product labels state that TSH is the biochemical endpoint to determine the thyroid hormone status. 5-10  The recommended management (under Laboratory Tests) is as follows: “The diagnosis of hypothyroidism is confirmed by measuring TSH levels using a sensitive assay….and measurement of free-T4.  The adequacy of therapy is determined by periodic assessment of appropriate laboratory tests and clinical evaluation.” 

The labels recommend testing in Adults, as follows: “The frequency of TSH monitoring during levothyroxine dose titration depends on the clinical situation but it is generally recommended at 6-8 week intervals until normalization.  For patients who have recently initiated levothyroxine therapy and whose serum TSH has normalized or in patients who have had their dosage or brand of levothyroxine changed, the serum TSH concentration should be measured after 8-12 weeks.  When optimum replacement dose has been attained, clinical (physical examination) and biochemical monitoring may be performed every 6-12 months, depending on the clinical situation, and whenever there is a change in the patient’s status.  It is recommended that a physical examination and a serum TSH measurement be performed at least annually…” 5-7

The product labels and professional societies recognize the importance of using TSH measurements as the endpoint for evaluating the biochemical thyroid status.  They state that once the patient is stabilized on an LT4 dose, periodic assessment needs only be done every six to twelve months. 5-8, 10 

4.2            LT4 therapy is individualized and carefully titrated

Treatment with LT4 products is individualized for each patient, based on their underlying thyroid status, age, and presence or absence of other clinical conditions, particularly their cardiac function.  With the exception of young healthy thyroid patients, the treated hypothyroid population is initiated with a low LT4 dose and titration of the dose is done in small increments until they are able to achieve their euthyroid state.  Thyroid cancer patients are carefully titrated to keep their TSH levels in the marginally hyperthyroid range.  Recalling the inverse log-linear relationship of TSH to T4 levels, titration to the marginally hyperthyroid state can require small dose increments.

In recognition of the narrow therapeutic window for serum T4 and TSH, and the log-linear relationship between TSH and T4 levels, professional societies and the product labels recommend that careful monitoring and titration be done when instituting LT4 therapy. 5-8, 10  Because of the cardiac and cardiovascular consequences of rapid replacement or over-replacement, definitive recommendations are provided for special patient populations.  Under Dosage and Administration – Special Patient Populations the product label recommends “For most patients older than 50 years of age or for patients under 50 years of age with underlying cardiac disease, an initial starting dose of 25-50 mcg/day of levothyroxine sodium is recommended, with gradual increments in dose at 6-8 week intervals, as needed.  The recommended starting dose for elderly patients with cardiac disease is 12.5-25 mcg/day, with gradual dose increments at 4-6 week intervals.  The levothyroxine sodium dose is generally adjusted in 12.5-25 mcg increments until the patient with primary hypothyroidism is clinically euthyroid and the serum TSH has normalized.” 5-7

The medical literature on which FDA based its decision to approve oral levothyroxine tablets uniformly emphasizes the clinical need for fine dosing increments.  As FDA stated in its review of Unithroid, “a 25 mcg dosage strength that meets chemistry and biopharm criteria for approval, is essential for proper labeling of the product for safe and effective use given that in certain clinical situations, levothyroxine sodium dosing is initiated at 12.5-25 mcg/day and increased in 12.5-25 mcg dosing increments.” 11

4.3            Fine-dosing increments – importance and medical use

The FDA recognizes that multiple dose strengths are required to accomplish adequate treatment of the thyroid patient population.  The FDA, in a final agency decision regarding the regulatory status of Synthroid®, emphasized that Patients Need a Precise Dose of Levothyroxine Sodium.  “The dosage of replacement therapy is increased in gradual increments until the TSH test indicates the correct maintenance dosage has been achieved.  In order to allow for fine adjustments of dose, which are necessary due to levothyroxine sodium’s narrow therapeutic range, levothyroxine sodium products are marketed in an unusually large number of dosage strengths.  Synthroid®, for example, comes in 25, 50, 75, 88, 100, 112, 125, 150, 175, 200, and 300 mcg strengths.” 12

Market research demonstrates that 1 in 5 dosage changes is an increase or decrease by 12 or 13 µg.

The impact on thyroid hormone status of small deviations from an optimally titrated dose is demonstrated in a prospective, longitudinal study by Carr et al.  Twenty-one patients on LT4 replacement therapy for hypothyroidism were studied while taking the dose that produced a normal TSH response, and then the patients were restudied at lower and/or higher doses (Figure 3). 14  Dosage changes of as little as 25 µg rendered the patients either hypothyroid or hyperthyroid, dependent upon the direction of the dose change from the dose that maintained them in a euthyroid state.


Figure 3.     Resultant TSH levels With Incremental 25 µg Changes in LT4 Dosage

Applying the inverse log-linear relationship of serum TSH to T4 levels, these data would predict that dose changes that were half those studied in the Carr study would also render some, if not all, of the patients outside of the normal TSH range (0.4 to 4.0 mIU/L). 

The ability to carefully titrate and maintain patients in the desired thyroid state is of paramount importance. The FDA acknowledged the same goals when approving the class labeling for all LT4 products.  In the product labels under Precautions it states, “Levothyroxine has a narrow therapeutic index.  Regardless of the indication for use, careful dosage titration is necessary to avoid the consequences of over- or under-treatment.  These consequences include, among others, effects on growth and development, cardiovascular function, bone metabolism, reproductive function, cognitive function, emotional state, gastrointestinal function, and on glucose and lipid metabolism.” 

4.4            Clinical consequences of hypothyroidism and hyperthyroidism

4.4.1              Patient populations treated with LT4 products

Functional thyroid disease can manifest as either over- or under-active thyroid hormone status. 15, 16  In either case, there is a wide spectrum of the clinical expression of the disease from mild to severe.  However, the clinical consequences of each are more severe the further the thyroid function has deviated from normal, i.e., severe thyrotoxicosis (severe hyperthyroidism) and myxedema coma (severe hypothyroidism).  Significant clinical consequences also occur with milder forms of the disease (“subclinical” thyroid disease) as may be seen when the patient is not treated to reach and maintain the euthyroid state.  In a large health screening study of 25,862 subjects in Colorado, 18% of all patients treated with LT4 products had TSH levels above the upper limit of the normal range, indicating that those patients were in a subclinical hypothyroid state despite LT4 treatment. 17

The American Cancer Society projects the number of new cases of thyroid cancer in 2003 will reach 22,000 with an annual mortality of 1,400. 18  The low mortality rates for this cancer is due in part to the effectivness care delivered for these patients.  Thyroid cancer patients undergo surgical removal of their thyroid gland and treatment with radioactive iodine to ablate the remaining thyroid cancer cells.  Thereafter, they are purposefully maintained in a marginally hyperthyroid state (TSH < 0.4 mIU/L). 5-7, 9, 19  TSH is a growth factor for normal and cancerous thyroid cells.  The goal of LT4 treatment is to deliver adequate LT4 to suppress the TSH to just below the normal range.  Lowering TSH levels removes the growth stimulus, thereby reducing the probability that any remaining thyroid cancer cells will grow to be of any clinical significance.  If these patients are under-treated, they are at risk of having a recurrence of their thyroid cancer or development of metastases.  Conversely, if they receive too much LT4 they are at risk of the complications of over-treatment, described below.

4.4.2              Consequences of hypothyroidism and hyperthyroidism

It is paramount that patients be guaranteed that any LT4 product substitution produce the same therapeutic response such that the efficacy and safety profile they rely upon is not compromised.

The FDA, in a final agency decision regarding the regulatory status of Synthroid, described the safety risks when patients are inadvertently over- or under-treated.  “Superpotent tablets of levothyroxine sodium pose safety risks.  Patients who inadvertently receive more levothyroxine than is necessary to control their condition may experience angina, tachycardia, or arrhythmias.  There is also evidence that overtreatment can contribute to osteoporosis.  Subpotent tablets of levothyroxine sodium are not adequately effective and, therefore, also pose safety risks.  Patients inadvertently receiving less than their proper dose may experience such symptoms as fatigue, lethargy, sleepiness, mental impairment, depression, cold intolerance, hair loss, hoarseness, weight gain, constipation, decreased appetite, dry skin, increased perspiration, arthralgia, menstrual disturbances, and paresthesias.  Because of the serious consequences of too much or too little circulating thyroxine, it is very important that patients receive the dose of levothyroxine sodium determined by their physicians to be optimal to replace the amount of hormone that would have been present naturally.” 12

The FDA stated that the potential side effects that occur with mild hypothyroidism and hyperthyroidism involve many different organ systems. 12  Some specific examples highlight the importance of maintaining thyroid hormones in their narrow therapeutic ranges.  Maternal thyroid hormone status, particularly during early pregnancy, is important to the well being of the pregnant woman’s offspring.  Early in pregnancy the fetus is totally dependent on receiving thyroid hormone from the mother. 20  Hypothyroidism during pregnancy has been associated with lower IQ scores in the children. 21

Mild thyroid failure is associated with elevated total cholesterol and LDL-cholesterol levels. 17, 22-26  This is consistent with the finding that thyroid hormone is a positive regulator for the production of LDL-receptors. 2  In the hypothyroid state removal of LDL-cholesterol particles from the plasma into the liver and other tissues would be limited.  Hypothyroidism is an independent risk factor for myocardial infarction. 27 

Hypothyroidism is associated with a slow heart rate (bradycardia) and decreased contractility of heart muscle. 28  Thyroid hormone-responsive genes have been identified that are consistent with these clinical findings. 1, 28  Clinical practice guidelines and product labels for LT4 products advise careful monitoring and treatment of thyroid disease patients who also suffer from heart failure, as both hypo- and hyperthyroidism can worsen the heart failure. 5-8, 10  As testing of cardiac function becomes more sophisticated, it is evident that even mild thyroid failure has a significant effect on cardiac muscle contractility. 29-31  In the hypothyroid patient, with each heartbeat, less blood is pumped from the heart to the rest of the body and increased backward pressure causes fluid to build up in the lungs and legs.  Hyperthyroidism is associated with rapid heartbeat (tachycardia) and atrial fibrillation. 28  Both of these also result in less blood being pumped to the rest of the body with each heartbeat.  Atrial fibrillation is also associated with an increased risk of stroke. 32

4.5            Summary of LT4 therapy and clinical consequences

In summary, physicians use the TSH levels to judge the adequacy of treatment.  To achieve treatment goals, physicians and the 13 million LT4 treated patients in the U.S. rely on multiple dosage strengths.  For titration and maintenance of the desired thyroid status, dosage strengths that differ by only 12 to13 µg are frequently used.  Changes in LT4 treatment or potency, with resulting changes in TSH levels, have significant clinical consequences. 

The concerns of the FDA as outlined above and recommendations in the LT4 product labels further emphasize the concern that LT4 product substitution should only be done when LT4 can be delivered so as not to produce over- or under-treatment.  This takes on particular significance for patients who have their TSH monitored every 6 to 12 months.  It is possible that LT4 product substitution could occur between two consecutive TSH assessments and the patient’s thyroid status could be changed toward under- or over-treatment.  It is these patients who have an increased health risk from a product that is supra- or sub-potent.

5.0                      Conclusions

The determination of bioequivalence of LT4 products should signify that, under all circumstances, these products are truly interchangeable without adverse clinical consequences, and without the need for clinical monitoring, retesting and retitration.  The goal of thyroid hormone replacement therapy for hypothyroid patients is to safety titrate the patient to the appropriate dose that achieves and then maintains the euthyroid state.  The goal of TSH suppression for the treatment of thyroid cancer is to remove the growth-promoting effect of TSH on thyroid cancer cells such that the patient does not suffer regrowth of the cancer.  For these clinical purposes, patients and their physicians rely upon serum TSH levels, the most sensitive and easily measurable parameter of thyroid hormone function.  To achieve the optimal TSH levels, physicians titrate individual patients using a wide range dosage forms, routinely using dosage increments in the 12-13 µg range.  The clinical evidence presented here demonstrates that small changes in T4 dosage result in TSH levels that are correlated with undesirable clinical consequences. 

The current FDA guidance for the assessment of LT4 bioequivalence does not account for endogenous thyroxine or its biologic regulation.  Results from the “bioequivalence” study (M02-417) reveal that LT4 products approved using the current FDA criterion and based on bioequivalence data without baseline correction for endogenous T4 levels could differ by as much as 33% from the reference product.  Furthermore, simple corrections of the T4 baseline did not sufficiently solve the problem, because two products that differed by 12.5% in thyroxine content would be declared bioequivalent.  Other methodological flaws were also observed that could further reduce the reliability of the current guidance to ensure that products declared bioequivalent will be substitutable in patients without adverse clinical consequences, and without the need to remonitor, retest and retitrate.

In summary, we recommend that the new data be taken into account and careful consideration be given to developing a specific guidance for the assessment of bioequivalence of levothyroxine sodium products.  To do so, this guidance must adequately consider the unique nature of the thyroid hormone system and the demonstrated limitations of the current criteria even with baseline correction.  Physicians and patients rely on dosage strengths that differ by only 12-13 µg.  The concerns of the FDA as outlined in the FDA final action for Synthroid® and recommendations in the LT4 product labels further emphasize the concern that LT4 product substitution should only be done when LT4 can be delivered so as not to produce over- or under-treatment.  This needs to be ensured because physicians and patients alike rely on receiving “the correct dose when filling and refilling their carefully calculated prescriptions.” 12  The necessity to deliver bioequivalent LT4 products assumes that the methodology used to determine bioequivalence is robust and sensitive enough to differentiate doses of LT4 that are truly different. 

6.0                      References

 

1.         Anderson GW, Mariash CN, Oppenheimer JH. Molecular actions of thyroid hormone. In: Braverman LE, Utiger RD, eds. Werner & Ingbar's The Thyroid: A Fundamental and Clinical Text. Philadelphia: Lippincott Williams and Wilkins, 2000:174-195.

2.         Bakker O, Hudig F, Meijssen S, Wiersinga WM. Effects of triiodothyronine and amiodarone on the promoter of the human LDL receptor gene. Biochem Biophys Res Commun 1998; 249:517-521.

3.         National Academy of Clinical Biochemistry. Laboratory medicine practice guidelines.  Laboratory support for the diagnosis and monitoring of thyroid disease, 2002. Vol. 2003: http://www.nacb.org/lmpg/thyroid_/mpg_pub.stm, 2003.

4.         Guidance for Industry. Levothyroxine sodium tablets - in vivo pharmacokinetic and bioavailability studies and in vitro dissolution testing. US Department of Health and Human Services, Food and Drug Administration, Center for Drug Evaluation and Research, February 2001.

5.         Levoxyl (levothyroxine sodium tablets U. Physician Package Insert. St. Louis, MO: Jones Pharma Incorporated (A wholly owned subsidiary of King Pharmaceuticals, Inc.), Revised Dec, 2001.

6.         Synthroid (levothyroxine sodium tablets USP). Physician Package Insert. Abbott Laboratories, North Chicago, IL.,. 03-5195-R1-Rev. July, 2002,.

7.         Unithroid (levothyroxine sodium tablets USP). Physician Package Insert. Corona, CA: Watson Pharma, Inc.  A Subsidiary of Watson Laboratories, Inc., Revised November, 2000.

8.         Singer PA, Cooper DS, Levy EG, et al. Treatment guidelines for patients with hyperthyroidism and hypothyroidism. JAMA 1995; 273:808-812.

9.         Singer PA, Cooper DS, Daniels GH, et al. Treatment guidelines for patients with thyroid nodules and well-differentiated thyroid cancer. Arch Intern Med 1996; 156:2165-2172.

10.       AACE Task Force. American Association of Clinical Endocrinologists medical guidelines for clinical practice for the evaluation and treatment of hyperthyroidism and hypothyroidism. Endocrine Practice 2002; 8:458-469.

11.       Unithroid. Medical Offiver Review, 2000.

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